This invention relates to a process of manufacturing diaphragms having a thickness of 0.3 to 3.0 mm and consisting of a wire net, preferably a nickel wire net, which serves as a carrier, and a porous ceramic layer having a thickness of 0.1 to 2.8 mm, which is joined to said wire net, preferably for electrolyses, wherein a layer consisting of a difficulty flowable metal powder, preferably a nickel powder, which consists of irregularly shaped particles, is applied to a support, the wire net is rolled or pressed onto the powder layer and the latter is compacted by 30 to 60% at the same time, and the metal powder is fired in an oxidizing atmosphere at temperatures of 800.degree. to 1500.degree. C. for 1 to 30 minutes, preferably 5 to 15 minutes.
Diaphragms for use in electrolyses should resist elevated temperatures and corrosion. They should not have an electron conductivity of their own and should have an adequate mechanical strength and their thickness should be minimized so that they have a very low resistance to the transport of electric charges in the electrolyte.
In order to accomplish this, EP-B 0 022 252 discloses a diaphragm which has a thickness of 0.3 to 0.7 mm and consists of porous sintered nickel, iron or copper and comprises a skeleton structure which is constituted by a wire net, preferably a nickel wire net, wherein the metal is oxidized at least in part to form metal oxide. To produce such a diaphragm a layer of the metal powder is applied to a wire net having a mesh opening size of 100 to 500 .mu.m in that a paste consisting of the metal powder and a binder or alcohol is spread on or sprayed onto the wire net and is compacted under a pressure of about 200 kg/cm.sup.2 and is simultaneously bonded to the wire net. The metal powder is subsequently subjected to a reducing sintering treatment at a temperature of 700.degree. to 1000.degree. C. for 10 to 20 minutes and to a succeeding oxidizing treatment at a temperature of 1000.degree. to 1200.degree. C. for up to 3 hours. Said processes can allegedly be carried out to produce diaphragms which have a large surface area and have a strength which is due to the fact that the oxidation is not excessive but a residual metallic structure is obtained. Because the formation of oxide proceeds from the surface throughout the entire body, the electrical resistance is sufficiently high. But it has been found that the diaphragms which have been described hereinbefore, particularly when they have large dimensions, do not have a constant strength, density and thickness throughout the entire body although a constant strength is required for ensuring that the surfaces of the diaphragms will resist an erosion by gas and liquid streams occurring in the cells for an electrolysis of aqueous solutions. A constant density and a constant thickness of the diaphragms are required for ensuring a uniform current density and an optimum purity of gas because a non-uniform current density, i.e., local concentrations of current, may result in local overheating and corrosive attacks, i.e., in a formation of holes in the diaphragms so that oxyhydrogen gas may be formed in the electrolysis of alkaline aqueous solutions.
It has been attempted to manufacture thin diaphragms having a constant strength, density and thickness in that nickel powder is strewn onto a support through a fine-mesh screen which extends over said support at a small distance therefrom, the strewed-on nickel powder layer is compacted by rolling and the nickel wire net is joined to the nickel powder layer at the same time. But said measures will not ensure a uniform distribution of the nickel powder on the support and thus the strength, density and thickness of the resulting diaphragm will not be uniform. Besides, when diaphragms having a large surface area are to be made the screen must be held at a uniform thickness from the support by means of spacers because without a provision of spacers the screen will be deflected by the nickel powder applied to the screen and under the pressure of the doctor blade which is moved over the nickel powder and the distance between the screen and the support will then be non-uniform. Moreover, spacers will form discontinuities in the nickel powder layer and the resulting gaps will strongly adversely affect the separation of the gas and the uniformity of the current flow.